The present invention is related to the field of fiberoptic networks and, more particularly, to noise reduction techniques for lasing optical amplifiers.
In fiberoptic networks and transmission systems, optical amplifiers are used to boost message signals. Lasing optical amplifiers, such as laser diodes or rare earth-doped fiber amplifiers, are often used. However, a problem which exists in all information transmission systems is noise. The noise figure of a fiberoptic amplifier is the ratio of the signal-to-noise ratio at the input in the frequency band of interest to the signal-to-noise ratio at the output in the same band, i.e., ##EQU1## where G is the amplifier gain. The noise figure is used to measure the amount of noise added by the amplifier. In lasing optical amplifiers, a large component of the noise generated within the amplifier is amplified spontaneous emission, or ASE.
One widely used technique to filter out the broad band ASE is the incorporation of a narrow linewidth optical band pass filter. This technique yields a typical noise figure reduction of 0.5 dB but suffers from a 3 dB insertion loss. Furthermore, this technique appears to be incompatible with wideband multi-signal systems.
In contrast, the present invention offers a fiberoptic amplifier system with a superior noise figure reduction.